Voltage Controlled Oscillators (VCO) find application in a variety of devices and systems such as cellular phones and automotive radar systems. VCO's can be configured to tune across a fairly wide frequency range using a relatively narrow control voltage range. However, the output of a typical VCO can vary with temperature. The frequency response of a VCO can be temperature compensated by applying a temperature variable voltage source to varactors that form part of a VCO resonant circuit. U.S. Pat. No. 7,164,325 discloses a temperature stabilised VCO where a reference end of the varactor can be supplied with a temperature dependent voltage source that has a temperature dependence that substantially compensates for varactor temperature dependence. A temperature-dependent voltage source can be configured, for example, as a Proportional-To-Absolute-Temperature module which can include a constant current source configured to drive a selectable resistive load which can include an array of selectable resistors. Such a linear temperature compensation scheme is generally adequate for long range automotive radar applications operating in the 76-77 GHz frequency band. However, more stringent requirements need to be met for short range systems (77-81 GHz). Non-linear temperature compensation systems are known. For example, U.S. Pat. No. 5,608,347 discloses a temperature-dependent signal compensation circuit for generating a dual sloped compensation signal responsive to changes in operating temperature of the compensation circuit. The compensated signal is used to vary the gain of an amplifier in this system. A switch selectively couples either a first slope calibration signal or a second slope calibration signal to an output voltage of a voltage generator circuit in response to the changes in operating temperature. For an automotive radar application where FMCW, (frequency modulated continuous wave) signals are employed, it is preferable for a non-linear compensation signal to be free of any discontinuities.